High voltage electrical power cables having at least one elongated metal element, such as metal tape, straps or wires, disposed around the cable insulation, either extending parallel to the cable axis or helically wound around the insulation, are well known in the art. Generally, such cables include a central stranded conductor with a semi-conducting shield therearound which is covered by a layer of insulation. Insulation shielding, in the form of a semi-conducting layer, is around the insulation, and the elongated metal elements are disposed around the insulation shield. A protecting jacket is disposed around the metal elements.
It is also known in the art that when the insulation of such cables is exposed to moisture, such as when they are installed underground, "electrochemical trees" are formed in the insulation which shorten the life of the cable.
Furthermore, attempts have been made to prevent the formation of such "trees" by introducing a sealant between the strands of the conductor and between the insulation shield and the metallic shielding tape. See U.S. Pat. Nos. 3,943,271 and 4,130,450. However, it has been found that the mere introduction of sealant into such spaces is not entirely satisfactory when the sealant is merely asphalt/rubber or a polyester compound which is not water swellable.
For example, voids may be formed in the sealant during the application thereof or may be formed when the cable is punctured accidentally. Furthermore, the components of such a cable, being made of different materials, have different coefficients of expansion, and the components are subjected to different or varying temperatures during manufacture, storage and/or operation of the cable which can cause the formation of voids.
In addition, the straps or wires are usually spaced from each other in the direction circumferentially of the insulation which can result in spaces between the straps or wires for the migration of moisture. When the tape is wound with the edge portions of the overlapping, there is a small space between the overlapping tape and the insulation shield adjacent to the edge of the underlying tape and there may be some spaces between the overlapping edge portions of the tape. If the tape is wound with slightly spaced edge portions, there are spaces between the edge portions for the migration of moisture. Even if it is intended that the tape, which is relatively thin, be wound with abutting edge portions, spaces between the edge portions do occur because of manufacturing difficulties and tolerances. Such spaces may not be completely filled by the sealant when it is applied, but even if they are, voids can develop at such spaces when the cable, or its components, is subjected to temperature changes.
Any such spaces or voids form locations for the ingress of moisture which can cause the formation of the deleterious "electrochemical trees" in the cable insulation, and the conventional sealants used in the cables, being unaffected physically by water, cannot eliminate such voids.